SQL - Structured Query Language
( a basic overview )
·
The backbone of any Query Based Language is
Relational Algebra which provides a concise , formal notation for
representation of queries . However , commercial database systems required a
query language which would be much user friendly and for this purpose SQL short
for Structured Query Language was created which was the most influential
commercially marketed query language .
·
SQL uses a combination
of relational algebra and relational calculus constructs for its construction
and implementation
·
Although we refer to
SQL language as a "query language" , SQL can do much more than
just query a database . SQL can define the structure of the data , modify the
data in the database and it can also specify security constraints . This means
that SQL can present to its user a brief highlight of the size of the database
that the user is working upon . SQL can also be used to define any database and
also the tables that a user wants to create and maintain over a database
residing in the system as well as server memory . Also , on top of that SQL can
be used to modify the structure of the database over which other data reside .
Also , SQL can be used to specify the various security constraints that need to
be defined over a database which would make the data specific to the users it
is meant for by provisioning the access specifiers for access limitation as
well as defining the needed integrity constraints which would ensure the ACID
properties of a suitable relational database management system .
·
In this article a complete User's Guide and
How to Use / Implement a SQL installation won't be discussed nor is it intended
for in this basic article but the aim of the article is to present SQL's
fundamental constructs and concepts for brief usage by any User who is not only
new to SQL but may be an intermediate or highly skilled versatile user of the
language .
·
Background of SQL -
IBM developed the original version of SQL which was originally called as
"Sequel" ( Any of the present day readers of the article can perhaps
think of the word Sequel as some sort of movie sequel kind of the unlimited
sequels of the Star Wars series .. seems a poor joke ) as part of the System R
project of the early 1970's . The Sequel language has evolved since then , and
its name has changed to SQL ( Structured Query Language ) . Many applications
and products in the form of standalone and server based products now implement
SQL as a part of their database engine and thus over the years SQL has become
the supreme rdbms package which is not only widely used but also the most user
friendly product among all the users of the current database users .
·
In 1986 , the American National Standards
Institute ( ANSI ) and the International Organisation of Standardisation ( ISO
) published an SQL standard which used to be called as SQL-86 at that point of
time ( the postfix 86 succeeding SQL obviously comes from the manufacturing
year of 1986 ) . Following this the standardisation organisation ANSI published
an extended standard for SQL , SQL -89 in the year 1989 . The next version of
the standard was then published in the year 1992 and thus the version was
therefore called as SQL-92 standard , and then later versions followed like
SQL-1999 etc . The most recent version that I have worked upon and also is
installed over my system is the SQL-2003 version which serves most of the
purpose . Apart from some server connection issues and importing data from
other incompatible transactional data formats , I have not faced any major
hurdles while using the present version of standalone SQL installation ,
However , if one wants to read a full scale bibliographic notes reference of
these standards , one may read them in the documentation notes of the
accompanying installation CD or executable software .
·
SQL language has the
several parts namely :
1) Data Definition
Language (DDL)
The SQL DDL provides commands for defining the
relation schemas , deletion of the relations and modification of the relation
schemas .
2) Interactive Data
Manipulation Language (DML )
The SQL DML
includes a query language based on both the relational algebra and the tuple
relational calculus . Which means that one can enter queries over a table and
find out the necessary information from the database . The search query is
mostly written in a language which implements the core
principles and working of relational algebra and
relational calculus as their underlying coding platforms
3) Integrity
The SQL DDL includes commands for specifying
integrity constraints that the data stored in the database must satisfy. All
the necessary updates that violate the integrity constraints are not allowed
over the query engine of SQL.
4) View Definition
The SQL DDL includes commands for defining the
views upon a database. Views are nothing but a sub-part representation of a
main table present over a database and it consists of only the structure of the
database with filtered or unfiltered data from the main table .However any form
of DML statements cannot be made to work upon a view.
5) Transaction Control
SQL includes commands for specifying the beginning
and ending of the transactions . The broader meaning of this statement says
about the way in which stored procedures , cursors and triggers could be used
for the sake of making and breaking any transaction at the desired point of
time
6) Embedded SQL and
Dynamic SQL
Embedded and Dynamic SQL defines the manner in
which SQL statements can be embedded within general purpose programming
languages such as C , C++ , Java , PL/I , Cobol , Pascal and Fortran .
7) Authorization
The SQL DDL includes commands for specifying access
rights to relations and views . This means that SQL provides its users with the
potential to access the tables present over a given / created / hosted database
specific rights for access and views to its data within the database . Many
database systems support most of the SQL-92 standards and some of the new
constructs which are present in SQL : 1999 and SQL : 2003 although no databases
provide non-standard features which defer from the detailed and provided
features of SQL as provided in the standard specification manual of the
different release versions of SQL .
